As is well known, polyblends of rubber with styrene/acrylonitrile polymers have significant advantages in providing compositions of desirable resistance to impact for many applications. Various processes have been suggested or utilized for the manufacture of such polyblends including emulsion, suspension and mass polymerization techniques, and combinations thereof. Although graft blends of a monoalkenyl aromatic and ethylenically unsaturated nitrile monomers and rubber prepared in mass exhibit desirable properties, this technique has had large scale commercial limitations upon the maximum degree of conversion of monomers to polymer which can be effected because of the high viscosities and accompanying power and equipment requirements, which are encountered when the reactions are carried beyond a fairly low degree of conversion after phase inversion takes place. As a result, techniques have been adopted wherein the initial polymerization is carried out in mass to a point of conversion at which the viscosity levels are still of practical magnitudes, after which the resulting prepolymerization syrup is suspended in water or other inert liquid and polymerization of the monomers carried to substantial completion.
N. E. Aubrey in U.S. Pat. No. 3,509,237 discloses a mass/suspension method of polymerization styrene/acrylonitrile having diene rubbers dissolved therein with the rubber being grafted, inverted and dispersed as rubber particles under agitation. After phase inversion, the viscous mixture is suspended in water and polymerization is completed producing a polyblend in the form of beads.
Such mass/suspension processes are used commercially, however, present the economic problems of batch operations requiring long cycles at relatively low temperatures to control the heat of polymerization. Continuous mass polymerization processes have great economic advantages if they can be run at higher temperatures and higher rates with the necessary control of the great heats of polymerization. In the case of polyblends, the dispersed rubber phase must be formed and stabilized as to its morphology bringing it through the continuous polymerization of the rigid matrix polymer phase so that the physical properties of the polyblend meet exacting property specifications.
N. E. Aubrey in U.S. Pat. No. 3,509,237 further discloses a process for preparing styrene/acrylonitrile/rubber polyblends having a first and second grafted rubber phase wherein the first grafted rubber has a large particle size and the second grafted rubber has a smaller particle size. Such polyblends have superior properties if the smaller particle size rubber phase constitutes the largest proportion of the total rubber phase. A process for making such polyblends is disclosed wherein the two grafted rubber polyblends are prepared in batch processes separately and thereafter melt blended mechanically to form a polyblend having a first and second grafted rubber phase.
Kydonieus et. al. discloses in U.S. Pat. No. 3,511,895 a continuous mass polymerization process for styrene-acrylonitrile monomer having a rubber dissolved therein wherein the polymerization is carried out continuously through three zones of polymerization for heat control. A diluent solvent is also used to reduce viscosities for mixing and pumping ease along with polymerization and heat control. Such processes require physically separated reactors providing different reacting conditions for each step of polymerization involving costly multiple reactors and auxiliary equipment. The ABS polyblend produced by the process has only a monomodal size distribution for the dispersed rubber particles which does not provide an optimum balance of toughness and gloss.
Heretofore, in the art of manufacturing ABS polyblends having matrix phase copolymers of monoalkenyl aromatic compounds and lower alkenyl nitrile compounds utilizing continuous mass polymerization, there has been a problem in controlling and achieving heat removal, particularly at higher rates of conversion and higher conversion levels. The problem arises because at high conversions conductive and convective heat transfer rates decline and become insufficient at high conversion rates to achieve uniformity of temperatures and conversion throughout a polymerizing mass which is constituted of monomers and polymers (and, possibly, other materials). If the heat transfer rate is not adequate, the conversion rate varies locally in the reaction mass, and consequently, the molecular weight distribution as well as the monomer composition changes in the matrix copolymer being formed. Consequently, in continuous mass polymerization processes known to the prior art, there has tended to be a practical upper limitation both on the percent conversion and on the rate of conversion achievable.
In such prior art processes, it has been conventional heretofore, in order to maintain, generally, uniformity of temperature, conversion and composition, to use a diluent. Diluents, however, suffer from the disadvantage that they must usually be removed from the polymerized product before such product is suitable for most end use purposes. Another disadvantage is that diluents tend to reduce the rate of polymerization (or polyrate), although, with a copolymer type such as styrene/acrylonitrile, theoretical maximum polyrates in mass polymerization are characteristically not achieved because of inherent heat transfer and mixing limitations in known equipment. From an efficiency standpoint, diluents can be regarded as materials which reduce the capacity of polymerization equipment in mass polymerization processes.
The present process overcomes the above problem of the prior art processes by providing a process for the continuous polymerization of ABS polyblends in a single reaction zone with excellent heat and compositional control providing uniform products of great commercial utility. A monoalkenyl aromatic and alkenyl nitrile monomer composition is continuously polymerized under steady state conditions. Vapor removal cooling and uniform mixing in said single reaction zone insures uniform temperatures and composition throughout the polymerizing liquid phase.
By the present invention for the continuous polymerization of ABS polyblends it is possible to achieve simultaneously (1) high rates of monomer to copolymer conversion, (2) a high total conversion of monomers to copolymer, and (3) a high uniformity of composition in the polymerizing mass of monomers, and copolymer, compared to the known prior art continuous polymerization processes for producing ABS polyblends having matrix copolymers of monoalkenyl aromatic and alkenyl nitrile monomers and containing dispersed crosslinked grafted rubbers. As a result, the polyblend matrix copolymers produced by the process of the present invention characteristically have a substantially constant molecular weight distribution and a substantially constant monomer composition. While ABS polyblends having matrix copolymers of monoalkenyl aromatic compounds and lower alkenyl nitrile compounds with such narrow distribution characteristics have been produced by batch processes, it is believed that such polyblends have never heretofore been produced or producable under steady state conditions using continuous mass polymerization at the conversion levels and rates of conversion achievable by utilizing the principles and practice of the present invention. Diluents may be utilized, but generally are not necessary or desirable in practicing the basic principles of the present invention.
In the practice of the process the present invention, it is possible to employ reflux condensation to control temperature and pressure in the reaction zone at substantially constant values. Indeed, substantially isothermal conditions prevail throughout the reaction zone. So far as can be determined from the prior art, it has never heretofore been possible to employ reflux condensation (or, broadly, vapor removal) in a continuous mass polymerization process for the manufacture of ABS polyblends having matrix copolymers of monoalkenyl aromatic compounds and alkenyl nitrile compounds without adversely broadening either or both the molecular weight distribution and the monomer composition in the resulting ABS polyblend product. Such a broadening of distributions is undesirable, generally speaking, because it reduces physical strength characteristics, increases the haze and the yellowness in the ABS polyblend, and thereby narrows the range of utilities for which the product copolymer is suitable. By the practice of the present invention, however, reflux condensation is used to make ABS polyblends having matrix copolymers having narrow compositional distributions.
In the practice of the process of the present invention, mixing conditions in the reaction zone are employed which maintain the polymerizing liquid phase of such zone during continuous mass polymerization in a substantially homogeneous condition at all times, independent of viscosity of the liquid phase of the reaction system in the reaction zone. Such homogeneity has been exceedingly difficult to attain in the prior art because of the characteristically high viscosities associated with high conversion rates and high conversion levels in polymerizing monoalkenyl aromatic compounds and lower alkenyl nitrile compounds. Even relatively small variations in compositional homogeneity of materials in the reaction zone have been found to adversely affect desired narrow molecular weight distribution and composition distribution in the resulting copolymer product. To gain such compositional homogeneity, in accordance with the practice of the present invention, it has been found preferable to employ a particular type of mixing action, as more particularly hereinafter explained and described. While especially useful with high viscosity reaction systems, this mixing action is suitable generally for the practice of the present invention over wide viscosity ranges.
It is an object of the present invention to provide a continuous mass polymerization process for producing ABS polyblends having matrix copolymers of at least one monoalkenyl aromatic compound and at least one alkenyl nitrile compound and having contained therein dispersed first and second grafted diene rubbers.
It is an object of the present invention to provide a process for producing ABS polyblends having matrix copolymers of monoalkenyl aromatic compounds and alkenyl nitrile compounds which have a substantially constant molecular weight distribution and a substantially constant monomer composition.
It is an object of the present invention to produce ABS polyblends wherein the matrix copolymers are polymerized at high monomer to polymer conversion rates and at high conversion levels.
It is an object of the present invention to produce ABS polyblends polymerized in a single reaction zone with continuous mass polymerization conditions using a substantially uniform composition distribution throughout the reacting mass of monomers and polymers.
It is an object of the present invention to produce ABS polyblends using vapor removal to remove heat of reaction and regulate temperature of such reacting mass.
It is an object of the present invention to utilize a combination of continuous reflux condensation and uniform, complete, continuous mixing in the continuous mass polymerization of ABS polyblends.
It is an object of the present invention to maintain substantially isothermal conditions in the reaction zone of the continuous mass polymerization process for ABS.
It is an object of the present invention to provide a process whereby one can easily and effectively manufacture ABS polyblends having azeotropic or non-azeotropic monoalkenyl aromatic alkenyl nitrile monomer compositions.
It is an objective of this invention to provide a process to prepare ABS polyblends of monoalkenyl aromatic and monoalkenyl nitrile copolymers as the rigid matrix phase having dispersed therein first and second grafted diene rubber particles having a bimodal particle size distribution.